Composite passive materials for ultrasound transducers

ABSTRACT

Provided herein are composite passive layers for ultrasound transducers having acoustic properties that can be easily tailored to the needs of the transducer application using current microfabrication techniques. In an embodiment, a passive layer comprises metal posts embedded in a polymer matrix or other material. The acoustic properties of the passive layer depend on the metal/polymer volume fraction of the passive layer, which can be easily controlled using current microfabrication techniques, e.g., integrated circuit (IC) fabrication techniques. Further, the embedded metal posts provide electrical conduction through the passive layer allowing electrical connections to be made to an active element, e.g., piezoelectric element, of the transducer through the passive layer. Because the embedded metal posts conduct along one line of direction, they can be used to provide separate electrical connections to different active elements in a transducer array through the passive layer.

FIELD OF THE INVENTION

The present invention relates to ultrasound transducers, and moreparticularly to composite passive materials for ultrasound transducers.

BACKGROUND INFORMATION

An ultrasound transducer is typically fabricated as a stack of multiplelayers that depend on the application of the transducer. FIGS. 1 a and 1b show typical ultrasound transducers. Each transducer comprises, fromthe bottom up, a backing layer 30, a bottom electrode layer 17, anactive element layer (e.g., piezoelectric element or PZT) 10, a topelectrode layer 13, a matching layer (or multiple matching layers) 20,and a lens layer (for focused transducers) 35 and 45. The lens may be aconvex lens 35 or a concave lens 45. The backing, matching and lenslayers are all passive materials that are used to improve and optimizethe performance of the transducer. The backing layer is used toattenuate ultrasound energy propagating from the bottom of thetransducer so that ultrasound emissions are directed from the top of thetransducer and the matching layer is used to enhance acoustic couplingbetween the transducer and surrounding environment. Different transducerdesigns (different sizes, frequencies, applications, etc.) requirepassive materials with different acoustic properties. Therefore, thereis a need for effective methods to control the acoustic properties ofthese materials to deliver consistent performance while maintainingmanufacturability and compliance with processing methods.

A common method to control the properties of passive layers is to adddifferent fillers in different quantities to an epoxy or polymer tocreate a matrix. Common filler materials include tungsten, alumina, andsilver (e.g., in powder form). For example, silver is used in very highquantities to make an otherwise insulating epoxy conductive. Tungstenand alumina are used to control the acoustic impedance of the passivelayer by varying the filler/epoxy matrix density. Although the method ofusing fillers has several advantages in terms of flexibility, simplicityand cost, it also has several drawbacks. This method can only raise theacoustic impedance up to a certain point after which the epoxy saturatesand will not mix with any additional filler. Also, the filler can movearound in the epoxy before the epoxy is cured, making it difficult tocontrol the final distribution of the filler in the epoxy. Anotherdrawback with tungsten and alumina is that the composite materialremains nonconductive. Another drawback is that changing the compositionof the passive layers in many cases also affects theirmanufacturability.

Some of these drawbacks can be overcome by adding more processing stepsor using novel mixing, casting and fabrication techniques. However,these techniques eliminate the main advantage of using filer/epoxymatrices, which is simplicity and flexibility.

Therefore, there is a need for passive layers and fabrication methodsthat provide high flexibility and manufacturability without sacrificingperformance or cost.

SUMMARY OF THE INVENTION

Provided herein are composite passive layers for ultrasound transducershaving acoustic properties that can be easily tailored to the needs ofthe transducer application using current microfabrication techniques.

In an embodiment, a passive layer comprises metal posts embedded in apolymer matrix or other material. The acoustic properties of the passivelayer depend on the metal/polymer volume fraction of the passive layer,which can be easily controlled using current microfabricationtechniques, e.g., integrated circuit (IC) fabrication techniques.Further, the metal posts provide electrical conduction through thepassive layer allowing electrical connections to be made to an activeelement, e.g., piezoelectric element, of the transducer through thepassive layer. Because the embedded metal posts in the exampleembodiment conduct along one line of direction, they can be used toprovide separate electrical connections to different active elements ina transducer array through the passive layer.

In an embodiment, a passive layer is fabricated by applying aphotoresist, e.g., using spin coating. Spin coating allows the thicknessof the photoresist to be precisely controlled by varying the viscosityof the photoresist and spin parameters. The photoresist is then exposedto UV light through a mask to transfer a pattern from the mask to thephotoresist. Portions of the photoresist are then selectively removed,e.g., using a developer, based on the pattern. Metal is then depositedin the areas where the photoresist has been removed to form the metalposts of the passive layer. Because the spacing, arrangement, anddimensions of the metal posts can be precisely controlled by the maskpattern, this fabrication method allows the metal/polymer fractionvolume, and hence acoustic properties of the passive layer to be easilycontrolled.

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features and advantages be includedwithin this description, be within the scope of the invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

In order to better appreciate the above recited and other advantages ofthe present inventions are objected, a more particular description ofthe invention briefly described above will be rendered by reference tospecific embodiments thereof, which are illustrated in the accompanyingdrawings. It should be noted that the components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereference numerals designate corresponding parts throughout thedifferent views. However, like parts do not always have like referencenumerals. Moreover, all illustrations are intended to convey concepts,where relative sizes, shapes and other detailed attributes may beillustrated schematically rather than literally or precisely.

FIG. 1 a shows a prior art ultrasound transducer comprising of a stackof layers with a convex lens.

FIG. 1 b shows a prior art ultrasound transducer comprising of a stackof layers with a convex lens.

FIG. 2 shows a transducer according to an exemplary embodiment of thepresent invention.

FIG. 3 shows a transducer according to another exemplary embodiment ofthe present invention.

FIG. 4 shows a transducer according to yet another exemplary embodimentof the present invention.

FIGS. 5 a-5 e show process steps for fabricating a transducer accordingto an exemplary embodiment of the present invention.

FIG. 6 shows a lead connected to a transducer according to an exemplaryembodiment of the present invention.

FIG. 7 shows an exploded view of a transducer array according to anexemplary embodiment of the present invention.

FIG. 8 shows an exploded view of a transducer array according to anotherexemplary embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 2 shows an exemplary ultrasound transducer 105 according to anembodiment of the invention. The transducer 105 comprises an activeelement 110, e.g., a piezoelectric element, and top and bottomelectrodes 113 and 117 deposited on the top and bottom surfaces of theactive element 110, respectively. The electrodes 113 and 117 maycomprise thin layers of gold, chrome, or other conductive material. Thetransducer's emitting face may have a square shape, circular shape, orother shape.

The transducer 105 further comprises a matching layer 120 on top of theactive element 110. The matching layer 120 comprises a plurality ofmetallic posts 123 embedded in a polymer matrix 127 or other material.The acoustic properties of the matching layer 120 depend on themetal/polymer volume fraction of the matching layer 120. Generally, theacoustic impedance increases for increases in the volume fraction ofmetal. For other materials, the acoustic properties depend on themetal/material volume fraction, where the material is the material inwhich the metal posts are embedded. As discussed below, themetal/polymer volume fraction can be easily controlled using currentmicrofabrication techniques, e.g., IC and MEMS fabrication techniques.Because the metal/polymer volume fraction can be easily controlled, theacoustic properties of the matching layer 120 can be easily tailored tothe needs of the transducer application using current fabricationtechniques. The transducer 105 also comprises a backing layer 130underneath the active element 110.

FIG. 3 shows an exemplary ultrasound transducer 205 according to anotherembodiment of the invention. Similar to the previous embodiment, thetransducer 205 comprises an active element 110, e.g., piezoelectricelement, and top and bottom electrodes 113 and 117 deposited on the topand bottom of the active element 110, respectively. The transducer 205also comprises a matching layer 220 on top of the active element 110.

The transducer 205 further comprises a backing layer 230 underneath theactive element. The backing layer 230 comprises a plurality of metallicposts 233 embedded in a polymer matrix 237 or other material. Theacoustic properties of the backing layer 230 depend on the metal/polymervolume fraction of the backing layer 230, which can be easily controlledusing current microfabrication techniques, e.g., IC and MEMS fabricationtechniques.

FIG. 4 shows an exemplary ultrasound transducer according to yet anotherembodiment of the invention. In this embodiment, the matching layer 320comprises a plurality of metallic posts 323 embedded in a polymer matrix327 or other material. Similarly, the backing layer 330 comprises aplurality of metallic posts 333 embedded in a polymer matrix 337 orother material.

Processing steps for fabricating a transducer according to an exemplaryembodiment will now be given with reference to FIGS. 5( a)-5(e). In thisexample, a matching layer is fabricated on the active element. However,it is to be understood that the processing steps can also be used tofabricate the backing layer or other passive layers of the transducer.

FIG. 5( a) shows an active element 110, e.g., a piezoelectric element,with top and bottom electrodes 113 and 117, e.g., gold on chromeelectrodes.

In FIG. 5( b) a layer of light-sensitive polymer or epoxy 427 is appliedon top of the active element 110 using spin or spray coating. Othercoating processes may also be used. In this example, spin coating isused to apply the layer of light-sensitive polymer or epoxy 427. Thepolymer or epoxy may be mixed with precursors and solvents to obtain adesired thickness. By varying the polymer or epoxy viscosity and thespin parameters, the coat thickness can be precisely controlled. Mostlight-sensitive epoxies and polymers are known as photoresists (e.g., UVcured epoxies) and they are classified as either positive or negativebased on their response to light. Positive photoresist becomes weakerand more soluble when exposed to light while negative photoresistbecomes stronger and less soluble when exposed to light. Photoresistsare commonly used in IC and MEMS fabrication with consistent repeatableresults.

In FIG. 5( c), a mask 460, e.g., chrome on glass, is used in conjunctionwith light exposure equipment to form a pattern in the photoresist 427.In this example, the photoresist 427 is positive and the mask 460 istransparent 462 in areas where the photoresist 427 is to be removed. UVlight 465 is filtered through the mask 460 and reaches the underlyingphotoresist 427. The areas of the photoresist 427 corresponding to thetransparent areas 462 of the mask 460 are exposed to the UV light 465.For the example of negative photoresist, the mask would be opaque inareas where the photoresist is to be removed.

In FIG. 5( d), the areas of the photoresist 427 that were exposed tolight are removed with a developer, e.g., solvent, leaving the desiredpattern imprinted in the photoresist 427. In FIG. 5( e), the metal posts423 are deposited on top of the active element 110 in the areas wherethe photoresist 427 has been removed. The metal posts 423 may bedeposited using sputtering, electroplating, or other metal depositionmethod. The metal may be nickel, silver, or other conductive material.The photoresist 427 and embedded metal posts 423 form the matching layer420.

The acoustic properties of the matching layer 420 depend on themetal/polymer volume fraction of the matching layer 420. Because thespacing, arrangement and dimensions of the metal posts 423 can betightly controlled using the above process steps, the metal/polymerfraction can be tightly controlled to obtain the desired acousticproperties of the matching layer 420 and optimize the transducer design.The pattern (opaque and transparent areas) of the mask determines thespacing, arrangement and dimensions of the metal posts, and hence themetal/polymer volume fraction. The above process can also be used tofabricate the backing layer to control the acoustic properties of thebacking layer, and other passive layers to control their acousticproperties.

Therefore, the above process provides an effective method to customizethe acoustic properties of passive layers for a particular transducerapplication. Further, the above process is compatible with currentfabrication methods, e.g. IC and MEMS fabrication methods.

Instead of the passive layer comprising the photoresist, the photoresistmay be removed, e.g., stripped off, after the metal posts are deposited.A polymer or epoxy may then be applied around the metal post to form thepassive layer. For the example of epoxy, the epoxy may be applied aroundthe metal posts, then cured and ground down to the desired passive layerthickness.

Other materials may be used to form the posts besides metal, includingnonconductive materials such as oxide, nitride, and the like. In thisexample, the acoustic properties of the passive layer depends on thevolume fraction of the post material to the polymer, e.g., photoresist,in the passive layer.

Metal posts embedded in a polymer matrix not only control the acousticproperties of the passive layer, but also make the passive layerconductive along one direction. A conductive passive layer isadvantageous in an ultrasound transducer because it simplifies theelectrical connections of the positive and/or negative leads to theactive element.

FIG. 6 shows an example of a lead 510 electrically connected to thebottom of the active element 110 through the backing layer 230, whichcomprises metal posts 233 embedded in a polymer matrix 237. In thisexample, the lead 510 may be connected to the backing layer 230, e.g.,by a conductive epoxy or solder 515, or laser fused to the backinglayer. A thin electrode layer 520 may be deposited on the bottom of thebacking layer 230 to facilitate the electrical connection. The lead 510may be part of a twisted pair wire or connected at the other end to acoaxial cable. A lead (not shown) may similarly be electricallyconnected to the active element through the matching layer.Alternatively, a portion of the matching layer may be removed to exposea small area of the top electrode 113, and the lead (not shown)connected directly to the top electrode 113.

Because the metal posts embedded in the polymer matrix are conductivealong one direction (thickness direction), the metal post can be used toprovide separate electrical connections to different active elements ina transducer array. This is advantageous over silver based conductiveepoxy, which cannot provide separate electrical connections.

The ability of the metal posts to provide separate electrical connectionin a transducer array is illustrated in FIG. 7. FIG. 7 shows an explodedview of an exemplary transducer array comprising two concentric activeelements 610 a and 610 b, e.g., piezoelectric elements PZTs. Thetransducer array may have more than two active elements.

The transducer array further comprises two electrodes 617 a and 617 b onthe bottom of the active elements 610 a and 610 b, respectively. Theelectrodes 617 a and 617 b are electrically isolated from each other andmay comprise thin layers of gold, chrome, or other metal deposited onthe active elements. The transducer array further comprises a backinglayer 630 comprising metal posts 633 a and 633 b embedded in a polymermatrix 637. The metal posts 633 b are aligned with the electrode 617 bwhile the other metal posts 633 a are aligned with the electrode 617 a.The number and arrangement of the metal posts shown in FIG. 7 areexemplary only. The backing layer 630 may comprise any number of postsin different arrangements. Further, the posts may have different shapesthan the ones shown in FIG. 7.

The transducer array also comprises electrodes 640 a and 640 b on thebottom of the backing layer 630. The electrodes 640 a and 640 b may beconnected to separate leads 650 a and 650 b, respectively, by conductiveepoxy, solder, or the like. The electrode 640 b aligns with metal posts633 b and electrode 617 b while the electrode 640 a aligns with metalposts 633 a and electrode 617 a. Thus, the electrode 640 b provides anelectrical connection to active element 610 b through metal posts 633 band electrode 617 b while the electrode 640 a provides an electricalconnection to active element 610 a through metal posts 633 a andelectrode 617 a. Therefore, the embedded metal posts 633 a and 633 benable separate electrical connections to different active elements 610a and 610 b in the transducer array through the passive layer 630. Thesame principle may be applied to the matching layer (not shown in FIG.7) to provide separate electrical connections through the matchinglayer. The separate electrical connections provided by the metal postallow the active elements in a transducer array to be independentlycontrolled and driven.

A passive layer comprising embedded metal posts can be used in othertransducer arrays having different configurations and sizes depending onthe application of the array. Examples of transducer arrays includelinear and annular transducer arrays, two-dimensional transducer arrays,and the like.

The advantages that transducers arrays provide in performance and beammanipulation generally come at the price of more complex electronics andcontrols for coordinating and driving the separate elements of thearrays. FIG. 8 shows an exploded view of an exemplary transducer array,in which electronics for controlling the elements of the array areprovided near the transducer array. The transducer array in FIG. 8 issimilar to the one in FIG. 7 except for an integrated circuit (IC) chip710 connected to the bottom electrodes 640 a and 640 b of the backinglayer 630. The IC chip 710 comprises metal contact pads 720 a and 720 bthat align with electrodes 640 a and 640 b, respectively. The electrodes640 a and 640 b may be bonded to the metal contact pads 720 a and 720 b,respectively, e.g., using solder bumps, to electrically connect the ICchip 710 to the transducer array. The IC chip 710 also comprises a metalcontact pad 730 to connect the IC chip 710 to an ultrasound system via acable, twisted pair wires, or the like. The electronics of the IC chip710 may be fabricated on a silicon substrate using standard CMOSmicrofabrication techniques.

In this embodiment, the IC chip 710 may contain electronics forindividually controlling and driving the active elements 610 a and 610 bof the array. For example, the electronics of the IC chip 710 maycomprise multiplexers and switches for selectively coupling a signal toone of the active elements. This advantageously reduces the number ofsignals that need to be transmitted over a cable to and from a remoteultrasound system. The unidirectional conduction of the metal posts 633b and 633 a allow the IC chip to individually address the activeelements 610 b and 610 a, respectively.

Instead of bonding the IC chip to the transducer array, the IC chip maybe located near the transducer array and connected to the transducerarray, e.g., by wires. For example, the IC chip and transducer array maybe mounted in the same housing next to each other. The IC chip may alsobe electrically connected to the transducer array through metal postsembedded in the matching layer as an alternative or in addition to thebacking layer. Further, the electronics of the IC chip may includefilters and processors for filtering and processing signals from thetransducer array before sending the signals over a cable to the remoteultrasound system.

Although metal posts were used in the preferred embodiment to provideconduction through the passive layer, other conductive materials may beused for the posts.

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Forexample, the reader is to understand that the specific ordering andcombination of process actions described herein is merely illustrative,and the invention can be performed using different or additional processactions, or a different combination or ordering of process actions. As afurther example, each feature of one embodiment can be mixed and matchedwith other features shown in other embodiments. Additionally andobviously, features may be added or subtracted as desired. Accordingly,the invention is not to be restricted except in light of the attachedclaims and their equivalents.

1. A method of fabricating a transducer, comprising: coating aphotoresist layer on an active acoustic element; exposing thephotoresist layer to light through a mask to transfer a pattern from themask to the photoresist layer; removing portions of the photoresistlayer based on the transferred pattern to create a plurality of voids inthe photoresist layer; and depositing conductive material in the voidsto form conductive posts embedded in the photoresist layer, wherein thepatterned photoresist layer and conductive posts form a passive layerattached to the active acoustic element.
 2. The method of claim 1,further comprising curing the photoresist layer after the conductiveposts are formed.
 3. The method of claim 1, wherein the active acousticelement comprises a piezoelectric element.
 4. The method of claim 1,wherein removing portions of the photoresist layer comprises removingportions of the photoresist layer that are oriented substantiallyperpendicular to an acoustic emitting face of the active acousticelement.
 5. The method of claim 1, wherein depositing conductivematerial comprises depositing a metal material in the voids to formconductive metal posts.
 6. The method of claim 5, wherein depositing ametal material comprises depositing a silver or nickel material in thevoids to form the conductive metal posts.
 7. The method of claim 1,wherein at least one of the conductive posts extends across a thicknessof the passive layer.
 8. The method of claim 1, wherein the passivelayer forms a matching layer that acoustically couples ultrasound energyfrom the active acoustic element.
 9. The method of claim 1, furthercomprising depositing an electrode on a surface of the passive layer,wherein the electrode is electrically coupled to the active acousticelement through at least one of the conductive posts.
 10. The method ofclaim 1, wherein coating a photoresist layer on an active acousticelement comprises coating the photoresist layer on a plurality of activeacoustic elements.
 11. The method of claim 10, further comprisingdepositing a plurality of electrodes on a surface of the passive layer;and electrically coupling each of the electrodes to one of the activeacoustic elements through at least one of the conductive posts.
 12. Themethod of claim 12, wherein electrically coupling each of the electrodescomprises electrically coupling each of the electrodes to a differentone of the active acoustic elements.
 13. The method of claim 10, furthercomprising electrically coupling an integrated circuit (IC) chip to atleast one of the active acoustic elements through at least one of theconductive posts.
 14. The method of claim 13, further comprising bondingthe IC chip to the passive layer.
 15. The method of claim 14, furthercomprising disposing the passive layer and plurality of acousticelements on the IC chip.
 16. The method of claim 13, wherein the IC chipcomprises a plurality of electrical contacts, and wherein electricallycoupling the IC chip to at least one of the active acoustic elementscomprises electrically coupling each one of the electrical contacts ofthe IC chip to a different one of the active acoustic elements in thetransducer array through at least one of the conductive posts.
 17. Themethod of claim 10, further comprising disposing a first array ofelectrodes between the passive layer and the plurality of acousticelements, wherein the first array of electrodes are electrically coupledto the conductive posts of the passive layer.
 18. The method of claim17, further comprising disposing a second array of electrodes on anopposite surface of the passive layer from the first array ofelectrodes, wherein the second array of electrodes are electricallycoupled to the first array of electrodes through the conductive posts ofthe passive layer.
 19. The method of claim 1, further comprisingdisposing a first electrode between the passive layer and the activeacoustic element and electrically coupling the first electrode to theconductive posts of the passive layer.
 20. The method of claim 19,further comprising disposing a second electrode on an opposite surfaceof the passive layer from the first electrode and electrically couplingthe second electrode to the first of electrode through the conductiveposts of the passive layer.